The present invention relates to a defrosting control for a heat pump and more particularly to a method of detecting frost generated on an outdoor side heat exchanger of an air-conditioner.
In general, when the outdoor temperature lowers as in winter while a heat pump is driven for heating a room, frost is generated on an outdoor side heat exchanger to cause the decrease in the heat exchange capacity of the outdoor side heat exchanger. This results in waste of electric power and a decrease in heating power. Consequently, frost on the outdoor side heat exchanger provides a serious disadvantage to the heat pump.
Under the circumstances, the refrigeration cycle is temporarily reversed to defrost the outdoor side heat exchanger, and the defrosting cycle is then switched to the heat pump to re-start the heating, such operations being carried out in repetition. There are known apparatuses for controlling such operations, which include a differential temperature detector-carrying defrosting apparatus adapted to detect the generation and nonexistence of frost on the basis of a difference between the temperature in the outdoor side heat exchanger and that of the outside air, and a mechanical timer-carrying defrosting apparatus adapted to detect the temperature in the outdoor side heat exchanger at predetermined time intervals.
In the case of the former apparatus, a differential temperature detector-carrying defrosting apparatus, defrosting is necessarily carried out every time when the temperature of the outside air decreases, so that the difference between the temperature in the outdoor side heat exchanger and that of the outside air reaches a preset level. Therefore, even when the humidity of the outside air is low with no frost generated on the outdoor side heat exchanger, the defrosting is started unnecessarily. In the case of the latter apparatus, a mechanical timer-carrying defrosting apparatus, heating is continued with the outdoor side heat exchanger being left not defrosted when this heat exchanger is in a nearly frosted state. Even when, in this case, frost generation starts on the outdoor side heat exchanger with the temperature of the outside air decreasing greatly, a defrosting operation is not started until a predetermined period of time has elapsed.
In order to eliminate such problems, as disclosed in Japanese Patent Publication No. 60-40774/1985, an attempt was made to start the defrosting when the temperature in an outdoor side heat exchanger (i.e., temperature of an indoor coil) is not higher than a preset level with a downward gradient of the temperature in the indoor side heat exchanger becoming steeper than a preset gradient.
If the defrosting is thus started, the condition of gradual formation of frost on the outdoor side heat exchanger in accordance with a decrease of the temperature in the indoor side heat exchanger is detected and, therefore, the formation and nonexistence of frost has been detected.
In the conventional defrosting control method as mentioned above, the defrosting of the outdoor side heat exchanger is done on the condition that the temperature in the indoor side heat exchanger is not higher than a predetermined level, so as to improve the accuracy of detecting the formation of frost on the outdoor side heat exchanger. Therefore, when the temperature in the indoor side heat exchanger is high, i.e., when this heat exchanger is operated in its sufficient capacity and fully exhibits its functions, an unnecessary defrosting operation (non-load defrosting) is not carried out. However, if another heater (for example, a stove) is in operation in the room in which this indoor side heat exchanger is installed, the temperature in this room becomes high due to the operation of the additional heater, so that the temperature in the indoor side heat exchanger also becomes high. Namely, even when frost is formed on the outdoor side heat exchanger with the functions of the indoor side heat exchanger not fully exhibited, the temperature in the indoor side heat exchanger becomes high, and the defrosting is not started, so that the outdoor side heat exchanger is covered with frost thicker and thicker in some cases. In such a case, the predetermined level referred to above may be set high. However, when the additional room heater is not provided in the same room (or when the heating capacity of an additional room heater operated in the room is small) with this predetermined level set high, the number of defrosting operations for a unit time increases accordingly, so that the frequency of non-load defrosting increases to cause the heating by the air-conditioner to be interrupted. Therefore, the predetermined level cannot be set high.